Carbohydrates

Carbohydrates (literally hydrates of carbon) are chemical compounds
that consist of monosaccharide sugars of varying chain lengths and that
have the general chemical formula Cm(H2O)n or are derivatives of such.
Certain carbohydrates are an important storage and transport form of
energy in most organisms including plants and animals. Carbohydrates
are classified by the number of sugar units into monosacchharides (e.g.
glucose), disaccharides (e.g. saccharose), oligosaccharides, and polysaccharides
(e.g. starch, glycogen, and cellulose).

Pure carbohydrates contain carbon, hydrogen, and oxygen atoms, in a
1:2:1 molar ratio, giving the general formula Cn(H2O)n. (This applies
only to monosaccharides, see below, although all carbohydrates have
the more general formula Cn(H2O)m.) However, many important "carbohydrates"
deviate from this, such as deoxyribose and glycerol, although they are
not, in the strict sense, carbohydrates. Sometimes compounds containing
other elements are also counted as carbohydrates (e.g. chitin, which
contains nitrogen).

The simplest carbohydrates are monosaccharides, which are small straight-chain
aldehydes and ketones with many hydroxyl groups added, usually one on
each carbon except the functional group. Other carbohydrates are composed
of monosaccharide units and break down under hydrolysis. These may be
classified as disaccharides, oligosaccharides, or polysaccharides, depending
on whether they have two, several, or many monosaccharide units.

Monosaccharides

Monosaccharides may be divided into aldoses, which have an aldehyde
group on the first carbon atom, and ketoses, which typically have a
ketone group on the second. They may also be divided into trioses, tetroses,
pentoses, hexoses, and so forth, depending on how many carbon atoms
they contain. For instance, glucose is an aldohexose, fructose a ketohexose,
and ribose an aldopentose.

Further, each carbon atom that supports a hydroxyl group (except for
the first and last) is optically active, allowing a number of different
carbohydrates with the same basic structure. For instance, galactose
is an aldohexose but has different properties from glucose because the
atoms are arranged differently.

The straight-chain structure described here is only one of the forms
a monosaccharide may take. The aldehyde or ketone group may react with
a hydroxyl group on a different carbon atom to form a hemiacetal or
hemiketal, in which case there is an oxygen bridge between the two carbon
atoms, forming a heterocyclic ring. Rings with five and six atoms are
called furanose and pyranose forms and exist in equilibrium with the
straight-chain form.

It should be noted that the ring form has one more optically active
carbon than the straight-chain form, and so has both an alpha and a
beta form, which interconvert in equilibrium. However, the carbohydrate
may further react with an alcohol to form an acetal or ketal, in which
case the two forms become distinct. This is the basic type of link between
the monosaccharide units of larger carbohydrates.

Disaccharides

Disaccharides are composed of two monosaccharide units bound together
by a covalent glycosidic bond. The binding between the two sugars results
in the loss of a hydrogen atom (H) from one molecule and a hydroxyl
group (OH) from the other.

The most common disaccharides are sucrose (cane or beet sugar - made
from one glucose and one fructose), lactose (milk sugar - made from
one glucose and one galactose) and maltose (made of two glucoses). The
formula of these disaccharides is C12H22O11.

Oligosaccharides and polysaccharides

Oligosaccharides and polysaccharides are composed of longer chains
of monosaccharide units bound together by glycosidic bonds. The distinction
between the two is based upon the number of monosaccharide units present
in the chain. Oligosaccharides typically contain between three and nine
monosaccharide units, and polysaccharides contain greater than ten monosaccharide
units. Definitions of how large a carbohydrate must be to fall into
each category vary however.

Oligosaccharides are found as a common form of protein posttranslational
modification. Polysaccharides represent an important class of biological
polymer. Examples include starch, cellulose and chitin. Table and powdered
sugar are some of the foods you find disaccharides in.

Strictly speaking, carbohydrates are not necessary for human nutrition
because proteins can be converted to carbohydrates—the traditional
diet of some peoples consists of nearly zero percent carbohydrate, and
they are perfectly healthy. However, carbohydrates require less water
to digest than proteins or fats and are an important source of energy.

The (very) low carbohydrate diet is infamous for producing “brain
fog” because your brain and central nervous system function almost
exclusively on glucose.

Some problems have been cited for the long term effects of a no-carbohydrate
diet for some individuals. Athletes, for instance, or those that participate
in high intensity activities, will have a considerable reduction in
performance, due to having little to no glycogen supplies stored in
muscle tissue. Additionally, nephrotoxicity may occur, particularly
in persons that are not very well hydrated.

Catabolism

There are three major metabolic pathways of carbohydrate catabolism:

1. Glycolysis
2. Citric acid cycle
3. Oxidative phosphorylation

Cellular function

Complex carbohydrates have been implicated in intercellular communication.
Because of their high variability and elaborate branched structures,
little has been understood about the method of action. In 2001, the
National Institutes of General Medicine (NIGMS) established a long-term
goal of understanding the underlying biological processes and awarded
roughly $34 million to a constortium of US universities.